This invention relates generally to operating combustion systems and, more particularly, to methods and systems for operating combustion systems to facilitate reducing NOx emissions.
Typical boilers, furnaces, engines, incinerators, and other combustion sources emit exhaust gases that include nitrogen oxides. Nitrogen oxides include nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). Total NO+NO2 concentration is usually referred to as NOx. Nitrogen oxides produced by combustion are mainly in the form of NO. Some NO2 and N2O are also formed, but their concentrations are generally less than approximately 5% of the NO concentration, which generally ranges from 200 to 1000 ppm for coal-fired applications. Nitrogen oxide emissions are the subject of growing concern because they are alleged to be toxic compounds and precursors to acid rain and photochemical smog, and contributors to the greenhouse effect.
Several commercial technologies are available to reduce NOx emissions from combustion sources. Currently, Selective Catalytic Reduction (SCR) is a commercial technology that is frequently used to facilitate NOx control. With SCR, NOx is reduced by reactions with Nitrogen Reducing Agents (N-agents, such as ammonia, urea, etc.) across the surface of a catalyst. Known SCR systems operate at temperatures of approximately 700° F. and routinely are able to achieve approximately 80% NOx reduction. However, several inherent drawbacks of SCR, and most importantly, its high cost, may prevent it from being an all-encompassing solution to the problem of NOx removal. Moreover, SCR requires the installation of a large amount of catalyst in the exhaust stream, and SCR catalyst life is limited. Specifically, catalyst deactivation, due to a number of mechanisms, generally limits catalyst life to about four years for coal-fired applications. Costs associated with system modifications, installation and operation, combined with the cost of catalyst material, render SCR quite expensive pollutant control technology. Furthermore, because the spent catalysts are toxic, the catalysts also present disposal problems at the end of lifetime.
To facilitate reducing costs compared to the SCR technology, the reaction of N-agents with NOx can proceed without a catalyst at a higher temperature. This process is called the Selective Non-Catalytic Reduction (SNCR). SNCR is effective over a narrow range of temperatures, or “temperature window” centered about 1800° F. wherein the N-agent forms NHi radicals that react with NO. Under ideal laboratory conditions, deep NOx control may be possible; however, in practical full-scale installations, the non-uniformity of the temperature profile, difficulties of mixing the N-agent across the full combustor cross section, limited residence time for reactions, and ammonia slip (unreacted N-agent) may limit SNCR's effectiveness. Generally, NOx control via SNCR is limited to between approximately 40% and approximately 50%. However, since SNCR does not require a catalyst and therefore has a relatively lower capital cost compared to SCR, it is a valuable option for NOx control with a lower efficiency of NOx control compared to SCR systems.
Other known combustion systems include combustion modifications such as Low NOx Burners (LNB), reburning, and over-fire air (OFA) injection control of NOx emissions via combustion staging. These technologies provide relatively moderate NOx control of between approximately about 30% and approximately 60%. However, their capital costs are low and, since no injection of N-agents is required, their operating costs are generally reduced in comparison to SCR or SNCR systems. NOx control in reburning is achieved by fuel staging wherein a main portion of the fuel, for example, approximately 80% to approximately 90% is fired through the conventional burners with a normal amount of air, for example, approximately 10% excess. A certain amount of NOx is formed during the combustion process, and in a second stage, the remainder of the fuel (reburn fuel) is added into the secondary combustion zone, called the reburn zone, to maintain a fuel-rich environment. The reburn fuel can be coal, gas or other fuels. In the reducing atmosphere within the fuel-rich zone, both NOx formation and NOx removal reactions occur. Experimental results indicate that within a specific range of conditions (equivalence ratio, temperature, and residence time in the reburn zone), NOx concentrations may typically be reduced by approximately 50% to approximately 60%. Part of the reburn fuel is rapidly oxidized by oxygen to form CO and hydrogen, and the remaining reburn fuel provides a fuel-rich mixture with certain concentrations of carbon-containing radicals: CH3, CH2, CH, C, HCCO, etc. These active species can either form NO precursors in reactions with molecular nitrogen or consume NO in direct reactions with it. Many elementary reaction steps are involved in NO reduction. The carbon-containing radicals (CHi) formed in the reburn zone are capable of reducing NO concentrations by converting it into various intermediate species with C—N bonds. These species, in turn, are converted into NHi species (NH2, NH, and N), which later react with NO to form molecular nitrogen. Thus, NO can be removed by reactions with two types of radicals, namely species: CHi and NHi. However, reactions of intermediate N-containing species with NO are typically slow in the absence of O2 and do not contribute significantly to NO reduction in the reburn zone. In the third stage OFA is injected to complete combustion of the fuel. Typically OFA is injected at a location where the flue gas temperature is about 1800° F. to about 2800° F. to facilitate achieving complete combustion. The temperature of the flue gas at a point where overfire air is injected is henceforth referred to as TOFA. The OFA added in the last stage of the process oxidizes remaining CO, H2, HCN, and NHi species as well as unreacted fuel and fuel fragments, to final products, which include H2O, N2, and CO2. At this stage, the reduced N-containing species react mainly with oxygen and are oxidized either to elemental nitrogen or to NOx. It is the undesired oxidation of N-containing species to NOx that limits the efficiency of the reburning process.
Generally, reburning fuel is injected at flue gas temperatures of about 2300° F. to about 3000° F. The efficiency of NOx reduction in reburning may increase with an increase in injection temperature because of faster oxidation of the reburning fuel at higher temperatures, resulting in higher concentrations of carbon-containing radicals involved in NOx reduction. For reburning fuel heat inputs up to about 20%, the efficiency of NOx reduction increases with an increase in the amount of the reburning fuel. With larger amounts of reburning fuel, the efficiency of NOx reduction flattens out and may even slightly decrease. Increasing residence time in the reburn zone also improves reductions in nitrogen oxides emissions by allowing more time for reburning chemistry to proceed.
Lastly, an Advanced Reburning (AR) process, which is a synergistic integration of reburning and SNCR, is also currently available. Using AR, the N-agent is injected along with the OFA and the reburning system is adjusted to facilitate optimizing NOx reduction with an N-agent. By adjusting the reburning fuel injection rate to achieve near-stoichiometric conditions, instead of fuel-rich conditions normally used for reburn, the CO level is facilitated to be controlled, and the temperature window for effective SNCR chemistry may be broadened. With AR, NOx reduction achieved from the N-agent injection is nearly doubled, compared with that of SNCR. Furthermore, with AR, the widening of the temperature window provides flexibility in locating the injection system and the NOx control should be achievable over a broad boiler operating range.
However, although the technologies described above are available and capable of reducing NOx concentrations from combustion sources, they are complex systems that are also expensive to install, operate, and maintain.